The major goals of this proposal are to delineate the structural motifs that are required for the checkpoint signaling functions of the ATM and Rad3-related protein, ATR and to elucidate the mechanisms of ATR recruitment to sites of DNA damage. ATR is a member of the phosphoinositide 3-kinase-related kinase gene superfamily that has been implicated as an essential regulator of mammalian DNA damage responses. ATR and the related protein ATM (ataxia-telangiectasia-mutated) are large molecular mass protein kinases that function atop signaling cascades that regulate cell cycle checkpoint activation, DNA repair, and transcriptional responses in the face of genetic damage. Recent advances have led to the identification of cellular substrates that are required for the checkpoint signaling functions of ATR and have pointed toward a critical role for ATR in the signaling of DNA damage incurred during S phase. Consistent with genome surveillance functions of ATR, recent results have shown that ATR targets to sites of DNA damage and/or stalled replication forks following cellular exposure to DNA damaging agents or DNA replication inhibitors. However, outside of its conserved carboxyl-terminal catalytic domain, the structural motifs that are required for the DNA damage-signaling functions of ATR have not been identified, nor have the underlying mechanisms of ATR regulation been elucidated. Within this context, we propose to: (1) delineate functional motifs that are required for the DNA replication checkpoint functions of ATR; (2) map and functionally characterize a nuclear foci-targeting domain in ATR; and (3) identify the cellular protein(s) that mediate the targeting of ATR to nuclear foci. The accomplishment of these objectives will be a strong first step toward understanding the mechanisms of ATR function and regulation. We hope that knowledge gained from these studies can be translated into a more fundamental understanding of how DNA damage is converted into cell regulatory cells, and ultimately, how genetic instability arises during tumor development.